Glass-to-metal seal



Gd, 8, 1940. H. SCOTT 2,217,423

GLASS-TO-METAL SEAL Original Filed April '50, 1936 Fig. 1.-

' X3 Alloy 1784 -Ll I L o 100 200 300 400 500C.

Empemi'ure LL! I l l 1 l 0 I00 200 300 400 50070.

WITNESSES:

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Patented Oct. 8, 1940 UNITED STATES PATENT OFFICE GLASS-TO-METAL SEALvania Application April 30, 1936, Serial No. 77,221 Renewed July 1'7,1940 2 Claims.

This application is a continuation-in-part of my application Serial No.376,291, filed July 5, 1929, now Patent 2,062,335 granted Dec. 1', 1936,and relating to seals between metal and glass.

My invention relates to vacuum-tight seals and it has particularrelation to seals between electrical insulating materials of the classtypified by glass and metallic compositions of the class typified byferrous base alloys.

For many applications of vacuum-tight joints of the character named, theuse of a hard or borosilica glass is of advantage. The high strain pointor temperature at which such a glass will first begin to soften permitsthe assembled apparatus to be treated out under high temperature vacuumconditions without danger of collapse. In cfiect, the strain pointrepresents the lower limit of the customary annealing range of theglass.

Iron-base alloys are of especial advantage in glass-sealing applicationsbecause of their low cost, easy fabrication, immunity from attack bymercury, and ability to wet glass. Such alloys may be compounded fromiron, nickel and cobalt to have mean expansivities very closely matchingthose of the described hard glass from a static or room temperature upto the inflection temperature of the alloy.

As above used, the term static designates the lower limit of thetemperature range to which the seal is in service to be subjected andthe term inflection designates the temperature above which theexpansivity of the alloy increases rapidly. The point of this abruptrise in coefiicient of expansion of the metal may, in the case of theferrous base composition above mentioned, be made to approach the strainpoint of commercially available grades of hard glass. It is only abovethe strain point that stresses from the differential expansion betweenthe metal and glass can be relieved substantially by plastic flow.

These iron-base alloys, however, are further characterized by havingexpansion-temperature curves which decrease in slope as the temperaturerises above room or static value and then start again to rise morerapidly as the infiection temperature is approached. The expansion curveof hard glass, on the other hand, is substantially linear in form withinthe range of temperatures below the strain point.

Typically, therefore, the curve of an iron-base alloy of comparable meanexpansivity will cross and recross that of hard glass. Such a conditionis contrary to the past accepted theory of glassto-metal sealsthat boththe glass and the metal should have the same coemcient of expansion foreach degree up to the annealing range of the glass. In the light of thistheory, others have predicted many practical difliculties in sealingferrous-base alloys into hard glass.

I have discovered that these difliculties are by no means insurmountableand that exact identity of expansion per degree up to the annealingrange of the glass is not necessary between the glass and the metal ifthe overall expansions of the materials between static or roomtemperature and the strain point of the glass are substantially thesame. In accordance with my discovery I am able to make satisfactoryseals between the two materials named.

My invention will best be understood through the following descriptionof two specific embodiments represented by Figs. 1 and 2 of theaccompanying drawing.

In Fig. 1, I have represented at 10 the expan- 2O sion-temperature curveof one composition of hard glass known as Meth and at 12 thecorresponding curve of a ferrous-base alloy which I have found toproduce a satisfactory seal with this glass. This alloy is composed of31.9% of nickel, 9.8% of cobalt, 0.79% of manganese, 0.01% of carbon andthe remainder of 57.4% of iron.

It will be noted that whereas curve I0 is substantially a straight line,curve I2 is sumciently irregular in shape to cross and recross at pointsM and i5, respectively. Between the strain point l8 of the glass and thestatic or room temperature, however, the overall expansivities of thetwo materials are substantially the same. The differences atintermediate temperatures are, I have discovered, insuflicient toproduce strains of an objectionable or disruptive nature.

In Fig. 2, I have represented a diiierent combination of sealablematerials. Curve 20 applies to G-BO glass composed of 72.4% S102; 10.2%B203; 9.8% NazC); 5.10% A1203; 1.75% PhD; and 0.40% K20.

Curve 22 applies to a ferrous-base alloy suitable for sealing into thisglass, which alloy is 5 made up 31.8% of nickel, 16.0% of cobalt, 0.65%of manganese, 0.01% carbon and the remainder of iron.

In the combination represented in Fig. 2, curve 1 22 also crosses curve20 of the glass at two points, 24 and 2B. The overall expansion,however, between the strain point 28 of the glass and the static or roomtemperature is substantially the same for the two materials and nodamaging stresses are set up at intermediate temperatures.

It will be obvious that other equivalent coinbinations 01' glass andferrous-base alloys complying with this condition may be successfullyJoined. My invention, therefore, is not to be re- 5 stricted exceptinsofar as is necessitated by the prior art and by the scope -01 theappended claims.

I claim as my invention:

1. A glass-to-metal thermal joint in which the 10 glass has an overallexpansion between the static temperature of the joint and the strainpoint of the glass substantially the same as that 01' the metal betweenthese temperatures and a coei'ficient of expansion substantiallydifl'erent from the metal at an intermediate temperature.

2. A vacuum-tight seal between a ferrous-base alloy and a glass havingan overall expansion between the static temperature of the seal and thestrain point of the glass substantially the same as that 01' the alloyfor the same range of temperatures and a thermal coeflicient ofexpansion diii'erent from that of the alloy at an inter mediatetemperature.

i HOWARD SCOTT.

